1. Field of the Invention
The present invention relates to an optimized imaging system for collection of high resolution imagery.
2. Description of Related Art
Imaging systems have been widely used to image objects at long distances using telescopes sensitive to various portions of the radiation spectrum.
A fundamental issue regarding such imaging systems is obtaining high resolution imagery. Resolution may be expensive to obtain or dangerous to collect, requiring large telescopes and/or close proximity to a potential threat. The spatial resolution inherent to an image collected by a given size telescope (e.g., based on aperture size) is determined by several factors including the wavelength of collected radiation, the dimensions of other apertures in the collection system including pixel aperture, geometric aberrations of the optical system, the spatial sampling of the image, and the stability of the collector's line of sight during image collections. Motion of the elements within the scene can cause smear, reducing the resolution of those elements unless the motion is captured in a fashion that improves how the object is sampled.
Various prior art systems have been designed to obtain imagery under various illumination conditions, including the daytime and nighttime. Obtaining high resolution imagery generally requires collecting well-sampled imagery at the shortest wavelengths of radiation possible. Prior art systems, for example, collect imagery in the daytime at spatial resolutions of inches to meters. These prior art systems utilize visible (VIS), near infrared (NIR) and medium-wave infrared (MWIR) (both reflective and thermal) regions of the spectrum to obtain daytime imagery. Prior art systems collect imagery at much lower spatial resolutions at nighttime than in daytime, by approximately a factor of 4-7 compared to daytime systems. Such prior art systems generally obtain imagery at night using MWIR (3.0 μm to 5.0 μm) and long-wave infrared (LWIR) (8.0 μm to 12.0 μm) sensors that primarily measure the thermal emissions of ambient scenes. Other prior art systems collect nighttime imagery from reflective VIS (0.3 μm to 0.7 μm), NIR (0.7 μm to 1.0 μm) and lower short-wave infrared (LSWIR) (1.0 to 2.0 μm) bands of the spectrum. A few prior art systems have obtained images of extremely hot or combusting objects at nighttime utilizing the panchromatic (PAN) (1.0 μm to 1.7 μm) and/or the LSWIR bands of the spectrum. However, images collected in the PAN waveband use intensifiers and/or designs with large pixels and are generally low in resolution. Resolution is a measure of being able to separate two or more objects in an image or to recognize an edge. Those skilled in the art recognize there is an inherent limitation to resolution for a given size telescope due to diffraction effects, though many imaging systems purposely degrade their resolution to enable wider area coverage or greater signal collection. A high resolution image, for example, is one where diffraction effects are the collecting dominant limitation to resolution.
Common knowledge instructs those skilled in the art to create sensors and systems sensitive to wavebands wherein one may find the greatest amount of useful energy to collect imagery at useful signal to noise ratios given the noise inherent to focal plane arrays and readout electronics. Accordingly, most prior art systems have focused on the VIS, NIR, MWIR, LWIR and LSWIR band to obtain imagery. These systems have generally taken advantage of the fact that more photons are available to impinge on sensors sensitive to these wavebands over other wavebands in the spectrum. Recently, prior art systems have been able to take advantage of airglow (e.g., chemiluminesence of the atmosphere) or moonglow (e.g., reflection of moonlight) to obtain degraded resolution images of distant objects.
In addition, common knowledge instructs those skilled in the art to create sensors and systems sensitive to the shortest wavelength possible while still being able to find sufficient useful energy to collect imagery. Finally, common knowledge instructs those skilled in the art to create sensors and systems where the atmosphere is transmissive at certain wavebands in the spectrum.
In part, due to such common knowledge, the upper short-wave infrared (USWIR, 2.0-2.6 um) waveband has not been widely used at night for imaging distant objects. Astronomy programs have used USWIR for study of distant stars and earth resource sensors have imaged the Earth in this band during the daytime. However, these long-range telescopes are generally required to be fixed on the distant object for great lengths of time to collect enough energy to obtain an image. In addition, the USWIR waveband has generally not been considered suitable for high resolution imaging at night as only very warm objects may be viewed at night unless a system has very degraded spatial resolution relative to the diffraction limit or collects energy for extensive (non-practical) periods of time. Consequently, it has been used for viewing combusting objects such as rocket plumes or for directly viewing man-made light sources. Moreover, technology limitations have steered research and development efforts away from systems utilizing the USWIR waveband.
Generally, the spectrum is categorized in terms of wavelengths, as may be seen from the values for each of the various wavebands. The LSWIR waveband, for instance, consists of wavelengths shorter than those of the USWIR waveband. In addition, as each of their names suggests, the LWIR, the MWIR, the USWIR and the LSWIR consist of wavelengths in the infrared part of the spectrum. The wavelengths decrease in length as one moves from the LWIR successively to the LSWIR part of the infrared spectrum.
A few prior art systems have combined data from two or more focal plane arrays to generate composite images. These prior art systems do not adapt collected image quality to ambient nighttime and/or daytime conditions. Moreover, these prior art systems do not combine nighttime images collected in the MWIR band with images collected at shorter wavelengths, provide high-resolution images of ambient temperature objects in the USWIR, collect high resolution images at night in the VIS, NIR, or LSWIR or collect high resolution images of both thermally emitted signals and reflected signals at night.